The invention relates to a contact assembly with a PTC thermistor.
For example, in the case of cooling units of refrigerators, co-called motor start-up PTC thermistors are connected in front of their electric motors, the drive shaft of which connects directly with the cooling compressor. In each start-up phase, of the electric motor, the resistance of the PTC thermistor increases from a few ohms, for example 10 .OMEGA. in the cold state, in a very short time, often in a few seconds measured from the cold state, to very high resistance values which, for example, are of the order of 20,000 .OMEGA..
Because of the very great increase in the resistance value of the PTC thermistor which takes place in a short time, it is always heated very strongly. The effect of these increases in temperature, repeated frequently in a comparatively short time, in particular in the case of PTC thermistors connected in front of cooling units, is that under unfavourable ambient conditions, for example oil or oily atmosphere, they are subjected to thermally induced ageing, leading to the occasional formation of cracks in the PTC thermistors, which may lead to fractures and therefore to destruction of the PTC thermistor.
In FIG. 3, a motor start-up PTC thermistor 3' of this type is held in a housing 1 by means of supports 2a, 2b. Electrical connections are generally provided on opposite sides of the PTC thermistor 3', in such a way that the current is preferably fed via spring contacts 5a' and 5b' which press against the side faces of the PTC thermistor 3' at points 3a', 3a" and 3b', 3b" which are exactly opposite one another. Electrical contact between the PTC thermistor 3' and the connections 6a' and 6b' is made in this way.
As described above, current is preferably supplied in the case of PTC thermistors of this type via spring contacts which bear with some pressure on the PTC thermistor. This entails the risk, for example, that if the PTC thermistor fractures then the current-carrying spring contacts can come into contact with one another, leading to a short-circuit, or that fairly sizeable fragments of a broken PTC thermistor may still be in contact with one another and currents measuring several amperes may flow through the fragments which are in contact with one another.
European reference EP 0 618 594 A1 discloses a contact assembly with PTC resistor. Given this arrangement, there is the risk that, even after a fracture of the PTC resistor, fragments will remain clamped between contact springs and prestressed parts, as a result whereof the flow of current in the fragments is not interrupted, so that it is entirely possible to start a smouldering fire which can under certain circumstances set the refrigerator on fire.